In electronic input scanners for document scanning, image information is acquired by sensing light from an image at an array of photosites arranged across a path of relative movement of the array and the image. The photosites, typically photodiodes or amorphous silicon sensors, are formed on a semiconductor substrate or chip, with one or more chips butted or otherwise arranged closely together to form the array. The photosite array may provide a 1:1 correspondence of photosites to the width of the actual image (a full width array), or may rely on optics to reduce the apparent image size to correspond to a smaller array. In use, each photosite produces an output signal related to the light intensity detected at the photosite.
The array of photosites is typically incorporated into what is known as a "charge-coupled device," or CCD. The CCD accumulates the signals from individual photosites in the array forming one scan line on the image. After the signals making one scan line are collected, the signals are loaded in parallel fashion to a shift register which typically forms part of the CCD. As each scan line of the image is scanned, the signals in the shift register are shifted out of the CCD in a linear fashion. Thus, the CCD accepts as an input the a set of light intensities within each scan line through an image, such as of a document to be copied, and produces as an output a time-varying linear signal corresponding to the various light and dark values within each scan line. This output is sent ultimately to a digital-to-analog converter, which converts the entire image into a quantity of digital data.
FIG. 1 shows a typical waveform which is output by a CCD. In this graph, voltage is shown as a function of time. The waveform is a sequence of varying voltage levels, each corresponding to one photosite in the scan line. The different levels are separated by reset pulses R of a regular frequency, which ideally are short pulses of a constant voltage. In a typical configuration of a CCD output, the value of the voltage level for each photosite varies according to the light level into the photosite: the brighter the input from the image to that particular photosite, the greater the difference between the output level and the voltage value of the reset pulses. The voltage level of the reset pulse is a fixed value from the value of the dark video (a condition in which no light is going into the photosite, as in a black area of an image). Because the difference in voltage values between the reset and the dark video is fairly constant, it can serve as an important reference with respect to the video.
In a typical application, the output of a CCD is a 100 to 400 mV signal on top of an offset ranging from 3 volts to 8 volts. It is significant that the offset voltage from various CCDs will vary widely among various types of CCDs and even among CCDs of the same type. Because of this variance in the voltage level of the offset, numerous prior art devices have attempted to normalize this offset so that the CCD waveform can be properly processed before being fed into a digital-to-analog converter. This normalization is crucial because the circuitry that will process the analog video requires a fixed, known offset in order to properly distinguish black video from white video.
A common technique for ensuring the proper normalization of signals from the CCD to the analog-to-digital converter is to employ an external source of DC between the CCD and the analog-to-digital converter so that the voltage of the offset may be adjusted accordingly. However, a common problem that occurs when such an external DC source is used is a drift in the voltage offset over a period of use. Such a drift will have conspicuous results in the quality of documents printed with the scanner. In the example waveform shown in FIG. 1, if the voltage of the reset pulses is equivalent to black and lower voltage levels correspond to relatively "lighter" parts of the image, a slow downward drift in the voltage offset will cause areas on the printed document to appear gray when they should be black, and generally lighten a document made from the scanned image. Further, when the drift occurs in the course of use, documents made according to the CCD output will show an unevenness in quality. If the drift is apparent within a single scan line, a copy made from the scanned image will be noticeably lighter towards one edge of the scan line. It is therefore an important concern in this art that the voltage level of the offset, which serves as a reference voltage for the system, be maintained at a constant value.
In the prior art, there have been numerous proposals for maintaining a constant voltage offset. U.S. Pat. No. 4,698,685 to Beaverson shows an arrangement which provides a gain correction for each pixel in an array. Gain values are stored for each pixel in an electronic storage device. As data is acquired by the array and directed to an image processor, each incoming pixel value is multiplied by a selected gain correction value to produce a gain corrected output. U.S. Pat. No. 4,639,781 to Rucci et al. shows that distortions in a video signal may be corrected by applying a continuous gain adjustment to the video information generated at the pixels and dynamically changing the gain factors on a line by line basis. U.S. Pat. No. 4,660,082 to Tomoshisa et al., teaches that calibration and shading correction of image data may be corrected in synchronism with input scanning by comparison to a density reference value. U.S. Pat. No. 4,216,503 to Wiggins shows deriving offset and gain values from the sensor, storing those values and subsequently using those values for signal correction. U.S. Pat. No. 4,314,281 to Wiggins et al. teaches providing a compensation signal compensating for variations in light to which the sensors are subjected and deriving the compensation signal over a group of pixels, by taking an average response from the group as the group is exposed to a test pattern. U.S. Pat. No. 4,602,291 to Temes teaches a multimode pixel correction scheme which includes correction for pixel offset and gain. U.S. Pat. No. 4,920,428 to Lin et al. teaches the use of an attribute value in conjunction with a gain or offset correction to determine a shift of the correction values, thereby increasing the effective range of correction. Although the above inventions each carry out their respective objects, all of them require relatively extensive ancillary elements in addition to the basic elements for CCD scanning, such as means for providing reference values, means for accumulating and averaging signal values, means for multiplying signals by a gain correction value, etc. The above inventions all address the problem of calibration; that is, they address the problem that, for a given light input, two pixels may produce different outputs. The above inventions all have the effect of performing mathematical manipulations on the analog data to achieve a desired result.
Of particular relevance to the present application is Japanese patent abstract 57-104368, which describes a circuit for correcting drift in a video signal. During the blanking period of the video signal (i.e., between the outputs of each scan line), a drift component is detected and fed back to a DC amplifier. One drawback of this invention is that, because the drift correction cycle occurs only after each scan of a line (as opposed to after the signal from each photosite), drift in the offset can still occur, with a noticeable effect on document quality, within each scan line. Japanese patent abstract 59-123374 describes a system for controlling video gain by clamping the input signal and a synchronizing part of the output signal which is amplified to a prescribed level by a variable amplifier. The value of the offset of the video signal is controlled by varying the gain of the variable amplifier-another form of using an external source of DC, in this case to correct drift which may well be caused by the external DC used for normalization. In contrast to these two patents, it is an object of the present invention to maintain the offset voltage of the sequence of CCD signals at a constant, normalized value without resorting to external sources of DC bias, which have been known to cause drift. In other words, the present invention described below circumvents a problem which prior art devices merely compensate for.
It is another object of the present invention to maintain the offset voltage of the sequence of CCD signals at a constant value while requiring only a relatively small number of additional parts.
It is another object of the invention to maintain the offset voltage of the sequence of CCD signals at a constant value by means of providing a control system which operates through the power return of an amplifier accepting the analog data in sequence.
Other objects will appear hereinafter.